Thursday, October 28, 2010

Hot Spot That Spawned Hawaii Was on the Move, Study Finds

The research by Tarduno and colleagues proves this notion wrong by showing that the bend is at least partly the result of the movement of the hot spot plume within the mantle itself.

"The Hawaii bend was used as a classic example of how a large plate can change motion quickly. You can find a diagram of the Hawaii-Emperor bend entered into just about every introductory geological textbook out there. It really is something that catches your eye," said Tarduno.

The new findings were published late last month on Science Express, the online research journal of the American Association for the Advancement of Science, and will be published in an upcoming issue of the print journal Science.

Michael Manga, a geologist at the University of California at Berkeley, said the findings by Tarduno and colleagues are believable and help resolve how the Hawaii bend was made possible.

"They've shown the way the Earth works is not inconsistent with the way we expect it to work," he said.

For years geologists have struggled to explain the dynamics of plate tectonics that accounted for the observed Hawaii bend. Given a fixed hot spot, models never could accurately explain the phenomenon, added Manga.

On the Move

To test the theory that the hot spot was on the move before the Hawaii bend, Tarduno and colleagues embarked on a two-month excursion July 1, 2001, aboard the research vessel JOIDES Resolution to collect samples of solidified lava flows from four seamounts. Back in their laboratories, the researchers analyzed the samples.

"The test is straightforward," said Tarduno, "You need to know how old the rocks are and where they formed."

Study co-author Robert Duncan, a geophysicist at Oregon State University in Corvallis, and colleagues looked at the decay of the element potassium to argon, which occurs at a constant rate, and determined that the seamounts located before the bend formed between 81 million and 47 million years ago.

Tarduno and his colleagues determined where the seamounts formed by analyzing a mineral called magnetite in the rock samples. As hot magma from an erupting volcano cools, magnetite's natural magnetization aligns with the Earth's magnetic pole, much like a compass needle. Once the magma hardens, this magnetization is locked in place, becoming a permanent record.

"Magnetite is like a little compass, it tells us the direction to the pole, and it tells us how far from the pole," said Tarduno.

If the hot spot had been fixed for the past 80 million years, the latitude as determined by the magnetite should be the same for each sample and should also be the same latitude as the current Big Island of Hawaii, according to the researchers.

Tarduno and colleagues found that the hot spot actually crept southward at a rate of about 1.6 inches (40 millimeters) per year between 81 and 47 million years ago. They write in their paper that the finding changes "our understanding of terrestrial dynamics."

Forty-seven million years ago, the southerly movement of the hot spot slowed drastically and maybe even stopped, said Tarduno. The result is the bend seen in the seamount chain, which until now was considered proof that the Pacific Plate itself had changed direction.

Research Implications

Given the fact that the Hawaii-Emperor Seamounts have been used as an example of plate motion change, Tarduno says the findings question whether or not major plates can actually undergo rapid changes in direction.

As well, if hot spots are not fixed, then they cannot be used as a fixed frame of reference to track things such as climate change over time. Additionally, the finding may force geologists to re-think how the tectonic plates have moved over time.

"It means you can't ignore the mantle in thinking about how plates are moving and thinking about dynamics of the plates," said Tarduno.